1. Field of the Invention
The present invention relates to solar cells and more particularly and process for interconnecting solar cell arrays in which the interconnections are made by way of a conductive epoxy patterned on the substrate which eliminates the need for wire bonding directly to the solar cells in order to minimize damage to the solar cells and reduce the cost for forming solar cell arrays.
2. Description of the Prior Art
Solar cells are known to be used in various applications for converting light energy to electrical energy. In fact, solar cell arrays are known to be used as the primary power source in space craft applications due to their self-generating the power by charging from sunlight. However, due to the relatively low energy output of such solar cells, a plurality of solar cells are formed in an array which, in turn, are interconnected with a relatively large number of additional solar cell arrays to form a solar panel that is capable of generating in an electrical power, for example to power a spacecraft.
A typical solar cell array module is about 6".times.12".times.3 mils. Typical solar cell panels can be as large as 14 feet.times.42 feet. Thus, a significant number of solar array modules must be interconnected to form a solar cell panel. In particular, individual solar cells are connected in series to provide a desired output voltage. The series blocks of solar cells in turn are connected in parallel to provide sufficient current output for the particular application.
Solar cells (also known as photo-voltaic cells) are known to be formed from semiconductor processing techniques. Such solar cells are typically formed as thin films on a substrate, typically 3 to 250 mils in thickness. Electrical contact pads are formed on the solar cells in order to enable the cell to be interconnected with other cells to form a solar cell array. Examples of solar cells are disclosed in U.S. Pat. Nos. 5,246,506; 4,892,592; 4,348,254; 4,240,842; 4,392,010; 5,963,790; 5,922,142 and 5,928,437, all hereby incorporated by reference.
Various techniques are known for interconnecting the solar cells to form solar cell arrays. Examples of such techniques are disclosed in U.S. Pat. Nos. 5,006,179; 5,011,544; 5,961,737; 5,620,528 and 5,021,099. However, there are problems with such techniques for interconnecting solar cells. In particular, U.S. Pat. Nos. 5,006,179; 5,961,737 and 5,620,528 disclosed interconnecting solar cells to form solar cell arrays by wire bonding electrical conductors to the wire bond pads on the solar cells. Such wire bonding is known to raise the temperature of the wire bond pads to 200.degree. C. or better which can cause damage to the solar cells. In addition, wire bonding techniques are known to result in short p-n junction of the solar cells due to the pressure from the wire bonder. Accordingly, alternate techniques for interconnecting solar cells have been developed. Examples of such alternate techniques are disclosed in U.S. Pat. Nos. 5,021,099 and 5,466,302, hereby incorporated by reference.
The '099 patent discloses an interconnection system in which the solar cells are interconnected by way of an interconnect circuit formed on a flexible dielectric substrate, disposed on the back side of the solar cell. The interconnection circuit is adapted to be used with solar cells having front mounted wire bonding pads. The interconnection circuit is formed with a plurality of vertically extending figures which extend in the direction generally perpendicular to the plane of the solar cell. The extending figures are disposed adjacent the edges of the solar cell and are bent to make electrical contact with the wire bond pads on the top side of the solar cell. Such a system, however, involves rather complicated processing techniques and can result in non-uniform contact pressure between the contact fingers and the wire bond pads on the solar cell, thus degrading the power output of the solar cell.
The '302 patent solves these problems by utilizing a electrically conductive paste in order to connect copper strips to the wire bond pads on the solar cell. Although the process disclosed in the '302 patent eliminates problems associated with soldering and solar cell degradation, the process disclosed by the '302 patent is relatively expensive and requires the formation of the copper strips for the interconnection.
As mentioned above, groups of solar cell arrays modules are interconnected to form solar panels. In order to avoid the interconnection problems discussed above, thermal compression bonding techniques have been developed for interconnecting solar cell arrays to form a solar cell panel. An example of such a technique is illustrated in FIGS. 1-5. Referring to FIGS. 1A and 1B, a conventional solar cell array is shown. In particular, a front or glass side 22 is illustrated in FIG. 1A, while a back or metal side 24 is illustrated in FIG. 1B. As shown in FIGS. 2A and 2B, various electrical ribbon conductors, identified with the reference numerals 26, 28, 30 and 32, are connected to the metal side 24 of the solar cell by conventional thermal compression bonding to form a solar cell assembly 34. The solar cell assembly 34 is assembled to a substrate 36, such as an aluminum hollow foil panel carrier, as illustrated in FIG. 3. A thermally conductive (but not electrically conductive) epoxy is patterned on the carrier 36 to form a number of interconnection pads 38, 40, 42, and 44 on the substrate 36. Subsequently, as illustrated in FIG. 4, the ribbon conductors 26, 28 and 30 on the solar cell assemblies 34 are aligned with the interconnection pads 38, 40, 42 and 44 on the carrier 36. As shown in FIG. 5, the solar cell assembly 34 is disposed on the carrier 36 with the electrical ribbon conductors 26-32 aligned with the interconnection pads 38-44 and secured thereto by thermal compression by way of a wire bonder. Unfortunately the wire bonder is in contact with the solar cells 20 during this process. The pressure from the wire bonder is known to short out the solar cells. Thus there is a need for a simplified method for interconnecting solar cell arrays which does not damage the solar cells.